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Intermittent stuttering

I have a 2002 subaru forester that will stutter and backfire without throwing a code, but not all the time. i have had it hooked up to a computer to check the O2 sensors and they are fine, it has a new Throttle position sensor, knock sensor, plugs, and catalytic converters (all put on in the last month). I have called a dealership and they have not heard of this happening, so this is my last resort.

It sounds like you’ve got the air path from air intake to the exhaust path in good shape. But there’s two other components, the fuel path, and the spark. hmm … backfiring is usually caused by excess fuel getting into the exhaust. Have you had the exhaust HC levels measured? That’s one idea.

Me, if it were my car and no codes and all the easy stuff had already been done – a visual check of all the vacuum hoses, spray carb cleaner in the air intake paths to see if it makes the rpm increase, make sure the egr valves functions normally, the pcv system is ok, spark at the spark plugs is good, compression ok, then I’d probably take a wild guess and think this is this is an intermittent spark problem. If one or more of the spark plugs don’t fire, that will allow unburned fuel into the exhaust stream. That should throw a misfire code in an ideal world. But it may happen infrequently enough the ECM doesn’t catch it. I’d suspect the crank angle sensor and cam angle sensors and coils and plug wires. Try replacing or moving these items around to other cylinders, other banks, etc, see if it makes a difference. It could of course be the ignition module itself. Or the ECM. A cracked solder joint, etc. Have you checked w/AllData , Consumer Reports Reliability Guide, module repair companies etc for known electronic module problems with this make/model/year?

Besides that, it could be a fuel problem too. A leaky or sticking-on fuel injector for example. Or the coolant temp sensor is faulty and the ECM thinks the coolant is cold when it is hot, so it spews more gas from the injectors than necessary. That should throw a rich code. But again it may happen too infrequently for the ECM to catch.

A good shop with the proper diagnostic eqp’t, like manuf scan tools, lab scopes, etc, should be able to figure it out, but not unless it happens reliably while the vehicle is at the shop. You might do some experiments on your own to see if you can find exactly what situations it will almost certainly happen, like certain ambient air temps, certain coolant temps, etc.

it almost seems like it isn’t getting enough gas. it only happens every now and then and then it will run fine for a short while again, lather rinse and repeat.

If you suspect a fuel problem and the fuel filter doesn’t need changing, then asking a shop for a fuel pressure test would be a good place start.

Get a can of Mass Airflow Sensor Cleaner from the auto parts store and use it to clean said sensor. Also dieconnect the sensor, check connector for corrosion and tightness. Maybe an intermittent loose connection.

It’s a long shot, but a can of MAF cleaner is cheap and can’t hurt.

Since it’s not throwing a code, I would buy a coil pack and try one cylinder at a time until you fix it. Seems intermittent coil packs are routine fix for this problem on the Subaru forums.

Fuel pressure test shows 8psi, i think the problem is the fuel pump, it should be 41-46psi

Yes, your are right, 8 psi won’t cut it on modern fuel injection systems. It has to be in the 30’s to the 40’s before the injectors spray fuel correctly. Besides the pump, a leaky injector could cause this too. But w/the symptoms you report, more likely the pump. You are pretty certain the fuel filter is ok, right? Be sure before replacing the pump to verify the fuel pressure regulator diaphragm hasn’t split. Remove the vacuum hose from it and see if there is any gasoline inside.

I had a mechanic do everything you listed and he said it has to be the pump.

If the fuel filter has been changed and the car only has 8 PSI of fuel pressure then the mechanic is right; the pump is probably the cause.

Another possibility could be if there was a poor wire connection causing a voltage drop to the pump which would make it run slower and cause excessively low fuel pressure.

I don’t think that’s the case here at all; just pointing out a very obscure and not likely what-if.

well, the fuel pump didn’t fix the problem. It stutters and won’t accelerate then it will take off for a second then do it all over again and then it will drive fine.

i have dumped over $1000.00 in parts and labor to still have this issue, i cant afford to keep this car.

Did he measure the pressure again? There is a fuel pressure regulator that controls fuel pressure.

Your symptoms sound similar to VW’s problems when they were having coil pack issues. It’s possible your coil pack is failing, but is not bad enough yet to set a misfire code. The symptom of a failing coil pack is intermittent random stumbling, just like you are experiencing.

Sometimes the problem is cracking in the plastic coil case that causes arcing. You might be able to see this happening by starting the engine in the dark, opening the hood, and spraying the coil with a mist of water. If the engine stumbles and you see electrical arcing, the water has found a crack in the coil.

I know you’ve thrown a lot of money at this, but I’d give it one more try by replacing the coil pack.

okay, the dealership found an O2 sensor and the coil pack and plug wires needed replaced. so total cash spent was $1500.00 in parts and labor.

Thanks for letting us know the fix.